Early Earth may not have been as hot as previously believed, according to researchers from Vanderbilt University in Tennessee. Conditions then may not have been completely inhospitable to life.

The Earth is believed to have been covered in lava during its first 500 million years, according to established thinking. This era is known to geologists as the Hadean Period, due to its resemblance to traditional Christian views of Hades.

Conditions on the Earth during its first half-billion years may have been more like the modern day than previously believed by geologists, according to the new study.

Zircon crystals formed four billion years ago were compared to more contemporary examples found in Iceland. That island nation exhibits geological conditions that many geologists compare to the Earth in its first 500 million years.  

"We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals -- and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments," Calvin Miller, Earth and Environmental Sciences professor at Vanderbilt University, said.

Geologists were unable to find rock formations from the Hadean Period, leading many scientists to believe our planet was covered in an ocean of red hot magma. Others stated frequent meteor strikes kept the surface of the planet molten. Each school of thought believed life was unable to survive the extreme conditions on the planet at the time.   

Zircon crystals from the Hadean Period were first discovered in sandstone deposits during the 1980s. These formations, normally associated with granite, provided geologists with their first opportunity to carefully study the crust of ancient Earth.

Radiometric dating was used to determine the age of the crystals, and other techniques provided information regarding the temperature and water content when the crystals formed. This research showed that the crust of the Earth during the Hadean Period was cool enough, at times, to support the presence of water. Some of these bodies may have been as large as oceans, according to the study.

Tamara Carley, a doctoral student at Vanderbilt University in 2009, collected thousands of crystals in Iceland for analysis. These structures were formed from the erosion of volcanoes over the last 18 million years. Carley found the crystals in Iceland formed at significantly higher temperatures, and in dryer conditions, than similar structures created during the Hadean Period.

Analysis of zircon crystals and what the findings could tell geologists about the distant past of the Earth was detailed in the journal Earth and Planetary Science Letters.